The Auk 111(4):917-932, 1994 INTRASPECIFIC VARIATION IN ELEPAIO FORAGING BEHAVIOR IN HAWAIIAN FORESTS OF DIFFERENT STRUCTURE ERIC A. VANDERWERF t Departmentof Zoology,University of Florida,Gainesville, Florida 32611, USA ABSTRACT.--Istudied intraspecificvariation in foraging behavior of an endemic, insectiv- orous bird, the Elepaio (Chasiempissandwichensis), in two Hawaiian foreststhat differed in degreeof human modification.The undisturbedforest had a closedcanopy, a denseunder- story, and a groundcover of native plants. The disturbed forest had much lower tree and shrub densities,and a ground cover of alien grasses.Search-and-attack rates, proportions of attackmaneuvers, and proportionalsubstrate use differed betweenhabitats. Birds in disturbed habitat attackedprey two-thirds as often as birds in undisturbedhabitat, hopped lessfre- quently, and flew farther and more often. They also did lessperch-gleaning and chasing, did more flight-gleaningand hawking,used small branches and the groundless often, and usedleaves and the air more often than birds in undisturbedhabitat. Disturbedareas may be lower-qualityforaging habitat because they require more difficult foragingmethods. Age was associatedwith variation in search-and-attackrates and proportionsof attackmaneuvers, but sexwas not. SubadultElepaio attacked prey lessoften than adults,searched more slowly, and usedsimpler maneuvers more often, possibly to compensatefor their lower proficiency. Log-linearanalysis showed that attackmaneuver was relatedto substrateand to tree species. Birdsperch-gleaned more often on twigs and in ohia (Metrosiderospolymorpha), hung more often on bark and in koa(Acacia koa), and flight-gleanedmore often on leaves.Elepaio showed muchflexibility in foragingbehavior and usedmore-diverse attack maneuvers and substrates than relatedcontinental species, which may allow Elepaioto exploitdisturbed habitats suc- cessfully.Received 2 August1993, accepted 11 January1994. STUDIESOF AVIAN foraging behavior tradi- may not be valid (Martin 1986, Grubb and tionally have focusedon niche partitioningand Woodrey 1990, Martin and Karr 1990). Recog- community structureand, hence, have empha- nition of the importance of intraspecificvaria- sized differencesamong species(e.g. MacAr- tion has led to the discoverythat foraging by thur 1958, Holmes et al. 1979, Szaro and Balda an individual bird is influenced by numerous 1979,Wiens and Rotenberry1981, Alatalo 1982). biotic and abiotic factors: age (reviewed in Speciesmay be separatedby one or more of Wunderle 1991); sex (Selander 1966, Peters and severalniche components, including plant spe- Grubb 1983, Petit et al. 1990);morphology (Fitz- cies,substrate, height, horizontal position,for- patrick 1985, Sherry 1985, Gustafsson 1988, aging technique,and foraging speed(reviewed Moermond 1990); intraspecific dominance in Airola and Barrett 1985, Martin 1986, Schoe- (Hogstad 1988,Grubb and Woodrey 1990);hab- ner 1986). itat structure (Maurer and Whitmore ! 981, Rob- This emphasison interspecificvariation often inson and Holmes 1982, Saboand Holmes 1983); obscureddifferences in foraging within a spe- food distribution and abundance (Holmes and cies (but see, for example, Partridge 1976, Schultz 1988);season and stageof breeding cy- Holmes et al. 1978, Gustafsson1988). Moreover, cle (Robinson1986, Hejl and Verner 1990,Sakai modelsof communitystructure based on niche and Noon 1990);weather (Grubb 1978);and time partitioning and speciesoverlap assumethat all of day (Verbeek 1972). individualsof a speciesforage identically, which Most studiesof intraspecificvariation in avi- an foraging have concentrated on one or two of the above factors, but a foraging bird re- sponds simultaneously to a complex and dy- • Presentaddress: Department of Zoology,Univer- namic set of stimuli. Ideally, one should ex- sity of Hawaii, EdmondsonHall, 2538 The Mall, Ho- amine all factors and their interactions at once, nolulu, Hawaii 96822, USA. but this is difficult and often impractical(Grubb 917 918 EmcA. V•Nr•ERWœR• [Auk,Vol. 111 1979). A reasonablecompromise is to examine METHODS a subsetconsisting of severalfactors that may interact. For insectivorous forest birds, habitat Studysite.--I conductedwork from Februarythrough structureis perhapsthe mostimportant deter- July 1991at HakalauForest National Wildlife Refuge minant of foraging behavior (Robinson and on the islandof Hawaii. The sitelies at approximately Holmes 1982), and several habitat variables con- 1,900 m elevation on the east side of Mauna Kea and stitute such a subset of related factors. is characterizedby sloping terrain, heavy rainfall (3 Habitat structure affects the abundance, dis- m/year), and daytimetemperatures rarely above 20øC. The natural vegetation type is montane rain forest. tribution, and perceptibilityof prey, and also For a detailed descriptionof the region, see Scottet which search-and-attack methods birds can em- al. (1986). ploy to captureprey (Fitzpatrick1980, Robinson Although the site is now protected, parts of the and Holmes 1982, Holmes and Schultz 1988). foresthave been extensivelymodified by human ac- Aspectsof habitat structure known to affect for- tivities during the last 100 years. Timber was har- agingof insectivorousforest birds include: plant vestedin someareas and large tractswere clearedfor species (Holmes and Robinson 1981, Franzfeb cattle ranching, resulting in a patchwork of highly 1983, Morrison et al. 1985); substrate (Jackson disturbed and relatively undisturbed forest. The 1979, Fitzpatrick 1980, Greenberg and Grad- boundary between these habitats is not sharp, and wohl 1980);structural characteristics of vege- even the "undisturbed" forest has been somewhat tation (Robinson and Holmes 1984, Whelan modified by the sameactivities. The undisturbed hab- itat consistedof closed-canopyforest with a moder- 1989); and foliage density (Maurer and Whir- ately denseunderstory and a groundcover primarily more 1981, Sabo and Holmes 1983). Valuable of native forbs and ferns. The disturbed habitat was insights on the effect of habitat structure on a shorter,open-canopy woodland with almostno un- foragingpatterns may be gainedby comparing derstoryand a ground cover of alien grassesintro- the behaviorof a single speciesin two habitats duced for cattle grazing. that differ in structure (Robinson and Holmes I selectedthree plots totaling 11.4ha in undisturbed 1982). Understanding effects of habitat struc- habitat and three totaling 10.6ha in disturbedhabitat ture on foragingmay be particularlyuseful in that representedextremes of the continuum from un- the conservationof specieswith restrictedhab- disturbedto disturbed.I choseplots in both habitats itat or specializedhabitat requirementsand in that were as close to each other as possible and at similar elevations(within 100 m) without including making managementdecisions that will alter areas that were intermediate in structure. habitat structure. Studyspecies.--The Elepaio is placedin a monotypic In Hawaii, human alteration of native habi- genus endemic to the Hawaiian Islands; its closest tats by physical disturbance, introduction of relativesare the monarchineflycatchers of Australasia alienspecies, and loss of nativespecies diversity and Oceania(Boles 1979, Pratt et al. 1987,Sibley and has been extensive (Kirch 1983, Stone and Scott Ahlquist 1990). Elepaio are common permanent res- 1985, Cuddihy and Stone 1990). Native Hawai- idents of both habitats at the study site, although ian birds appear to be especiallysensitive to population density is higher in undisturbedhabitat disturbance(Olson and James1984, Sakai 1988), (unpubl. data). They are monogamousand nonmi- and many specieshave restrictedranges or are gratory, remaining paired and territorial throughout the year (MacCaughey1919, Conant 1977, Berger 1981). less abundant in disturbed areas (Scott et al. Elepaio are insectivorous(Munro 1960, Conant 1977, 1986). The Elepaio (Chasiempissandwichensis) is VanderWerfunpubl. data),although nectarivoryhas an insectivorous bird endemic to the Hawaiian been reported (MacCaughey1919). Islandsthat inhabitsforests of varying structure I was able to sexand age Elepaioby plumage dif- and degree of human modification. ferences(MacCaughey 1919, Pratt 1980). Males have The goalsof my study were to: (1) compare black throat feathers narrowly tipped with white, foraging methodsused by Elepaio in undis- whereas females have broader white tips that often turbed and human-modified forests to under- makethe throat appearalmost completely white. Sub- standhow they forageand whether humandis- adult birdsof both sexesretain a uniformly drabgray- turbanceaffects their ability to exploit a habitat; brown plumage for at least one year. All subadult Elepaio I observedwere second-yearbirds and were (2) investigatediversity and degree of intra- no longer being fed by adults. specificvariation in Elepaioforaging behavior; I used four methodsto identify 48 individual Ele- (3) simultaneously examine effects of several paio in the study plots: a unique combination of col- habitat variableson foraging to evaluatepos- ored leg bands(n = 17); distinctiveplumage (n = 5); sible interactions between factors. being paired with a color-bandedor distinctively- October1994] ElepaioForaging Behavior 919 plumagedbird (n = 10);and known territory bound- ward hover-glean"and "strike" (Fitzpatrick 1980), aries (n = 16). Based on observations of banded birds, "hover," and "snatch" (Robinson and Holmes 1982, it was unusualto find a bird in a territory that